Planar waveguide reflective diffraction grating

ABSTRACT

The invention relates to a planar waveguide reflective diffraction grating for use in an optical device, such as a wavelength division multiplexer, providing an increased bandwidth over conventional planar waveguide reflection diffraction gratings while eliminating the polarization dependent loss (PDL) typically associated therewith. Accordingly, a low order (&lt;3), high aspect ratio (&gt;10) grating is provided with a very short side wall (less than the wavelength of the optical signal) for use with incident angles of less than 15°.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. patent application Ser.No. 60/555,697 filed Mar. 24, 2004, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a reflective diffraction grating foruse in optical communication, and in particular to a planar waveguidereflective diffraction grating device with reduced polarizationsensitivity and an increased free spectral range.

BACKGROUND OF THE INVENTION

In optics, a diffraction grating is an array of fine, parallel, equallyspaced grooves (“rulings”) on a reflecting or transparent substrate,which grooves result in diffractive and mutual interference effects thatconcentrate reflected or transmitted electromagnetic energy in discretedirections, called “orders,” or “spectral orders.”

The groove dimensions and spacings are on the order of the wavelength inquestion. In the optical regime, in which the use of diffractiongratings is most common, there are many hundreds, or thousands, ofgrooves per millimeter.

Order zero corresponds to direct transmission or specular reflection.Higher orders result in deviation of the incident beam from thedirection predicted by geometric (ray) optics. With a normal angle ofincidence, the angle θ, the deviation of the diffracted ray from thedirection predicted by geometric optics, is given by the followingequation, where m is the spectral order, λ is the wavelength, and d isthe spacing between corresponding parts of adjacent grooves:$\theta = {\pm {\sin^{- 1}( \frac{m\quad\lambda}{d} )}}$

Because the angle of deviation of the diffracted beam iswavelength-dependent, a diffraction grating is dispersive , i.e. itseparates the incident beam spatially into its constituent wavelengthcomponents, producing a spectrum.

The spectral orders produced by diffraction gratings may overlap,depending on the spectral content of the incident beam and the number ofgrooves per unit distance on the grating. The higher the spectral order,the greater the overlap into the next-lower order. Diffraction gratingsare often used in monochromators and other optical instruments.

By controlling the cross-sectional shape of the grooves, it is possibleto concentrate most of the diffracted energy in the order of interest.This technique is called “blazing.”

Originally high resolution diffraction gratings were ruled. Theconstruction of high quality ruling engines was a large undertaking. Alater photolithographic technique allows gratings to be created from aholographic interference pattern. Holographic gratings have sinusoidalgrooves and so are not as bright, but are preferred in monochromatorsbecause they lead to a much lower stray light level than blazedgratings. A copying technique allows high quality replicas to be madefrom master gratings, this helps to lower costs of gratings.

A planar waveguide reflective diffraction grating includes an array offacets arranged in a regular sequence. The performance of a simplediffraction grating is illustrated with reference to FIG. 1. An opticalbeam 1, with a plurality of wavelength channels λ₁, λ₂, λ₃ . . . ,enters a diffraction grating 2, with grading pitch Λ and diffractionorder m, at a particular angle of incidence θ_(in). The optical beam isthen angularly dispersed at an angle θ_(out) depending upon wavelengthand the order, in accordance with the grating equation:mλ=θ(sinθ_(in)+sinθ_(out))  (1)

From the grating equation (1), the condition for the formation of adiffracted order depends on the wavelength λ_(N) of the incident light.When considering the formation of a spectrum, it is necessary to knowhow the angle of diffraction θ_(Nout) varies with the incidentwavelength θ_(in). Accordingly, by differentiating the equation (1) withrespect to θ_(Nout), assuming that the angle of incidence θ_(in) isfixed, the following equation is derived:∂θ_(Nout)/∂λ=m/Λ cos θ_(Nout)   (2)

The quantity dθ_(Nout)/dλ is the change of the diffraction angleθ_(Nout) corresponding to a small change of wavelength λ, which is knownas the angular dispersion of the diffraction grating. The angulardispersion increases as the order m increases, as the grading pitch Λdecreases, and as the diffraction angle θ_(Nout) increases. The lineardispersion of a diffraction grating is the product of this term and theeffective focal length of the system.

Since light of different wavelengths λ_(N) are diffracted at differentangles θ_(Nout), each order m is drawn out into a spectrum. The numberof orders that can be produced by a given diffraction grating is limitedby the grating pitch Λ, because θ_(Nout) cannot exceed 90°. The highestorder is given by Λ/□_(□). Consequently, a coarse grating (with large Λ)produces many orders while a fine grating may produce only one or two.

The free spectral range (FSR) of a diffraction grating is defined as thelargest bandwidth in a given order which does not overlap the samebandwidth in an adjacent order. The order m is important in determiningthe free spectral range over which continuous dispersion is obtained.For a given input-grating-output configuration, with the gratingoperation at a preferred diffraction order m for a preferred wavelengthλ, other wavelengths will follow the same path at other diffractionorders. The first overlap of orders occurs whenmλ_(m)=(m+1)λ_(m+1)  (3)$\begin{matrix}{\lambda_{m + 1} = \frac{m\quad\lambda_{m}}{( {m + 1} )}} & (4) \\{{\Delta\lambda} = \frac{\lambda_{m}}{m + 1}} & (5)\end{matrix}$

A blazed grating is one in which the grooves of the diffraction gratingare controlled to form right triangles with a blaze angle w, as shown inFIG. 1. The selection of the blaze angle w offers an opportunity tooptimize the overall efficiency profile of the diffraction grating,particularly for a given wavelength.

Planar waveguide diffraction based devices provide excellent performancein the near-IR (1550 nm) region for Dense Wavelength DivisionMultiplexing (DWDM). In particular, advancements in Echelle gratings,which usually operate at high diffraction orders (40 to 80), high anglesof incidence (approx 60°) and large grading pitches, have lead to largephase differences between interfering paths. Because the size of gratingfacets scales with the diffraction order, it has long been consideredthat such large phase differences are a necessity for the reliablemanufacturing of diffraction-based planar waveguide devices. Thus,existing devices are limited to operation over small wavelength rangesdue to the high diffraction orders required (see equation 5).

Furthermore, for diffraction grating-based devices fabricated in aplanar waveguide platform, a common problem encountered in the prior artis polarization dependent loss arising from field exclusion of onepolarization caused by the presence of conducting metal S (a reflectivecoating) adjacent to the reflective facets F.

An optical signal propagating through an optical fiber has anindeterminate polarization state requiring that the (de)multiplexer besubstantially polarization insensitive so as to minimize polarizationdependent losses. In a reflection grating used near Littrow condition,and blazed near Littrow condition, light of both polarizations reflectsequally well from the reflecting facets (F in FIG. 1). However, themetalized sidewall facet S introduces a boundary condition preventinglight with polarization parallel to the surface (TM) from existing nearthe surface. Moreover, light of one polarization will be preferentiallyabsorbed by the metal on the sidewall S, as compared to light of theother polarization. Ultimately, the presence of sidewall metal manifestsitself in the device performance as polarization-dependent loss (PDL).

There are numerous methods and apparatus for reducing the polarizationsensitivity of diffraction gratings. Chowdhury, in U.S. Pat. Nos.5,966,483 and 6,097,863 describes a reduction of polarizationsensitivity by choosing to reduce the difference between first andsecond diffraction efficiencies of a wavelength within the transmissionbandwidth. This solution can be of limited utility because it requireslimitations on election of blaze angles and blaze wavelength.

Sappey et al, in U.S. Pat. No. 6,400,509, teaches that polarizationsensitivity can be reduced by including reflective step surfaces andtransverse riser surfaces, separated by a flat. This solution is also oflimited utility because it requires reflective coating on some of thesurfaces but not the others, leading to additional manufacturing stepsrequiring selective treatment of the reflecting interfaces.

The free spectral range of gratings is proportional to the size of thegrating facets. It has long been thought that gratings with a smalldiffraction order could not be formed reliably by means ofphotolithographic etching, because low order often implies steps smalleror comparable to the photolithographic resolution. The photolithographicresolution and subsequent processing steps blur and substantiallydegrade the grating performance. Therefore, the field of etched gratingshas for practical reasons limited itself to reasonably large diffractionorders typically in excess of order 10. Devices with orders rangingclose to order 1 have long been thought to be impractical to realize.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing a planar waveguide reflective diffractiongrating providing an increased bandwidth, due to operating at arelatively low order, with very little PDL, due to very small sidewalllengths.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a reflective diffractiongrating device on a planar waveguide platform for use in demultiplexingan optical signal, comprising:

an input port for launching a beam of light comprising a plurality ofwavelength channels, defined by an average wavelength, at a diffractiongrating incident angle;

a reflective diffraction grating for dispersing the wavelength channelsat various angles according to wavelength, the reflective diffractiongrating having a plurality of reflective walls defined by a facetlength, and a plurality of sidewalls defined by a sidewall length; and

a plurality of output ports positioned to capture the wavelengthchannels;

wherein an aspect ratio, defined by the facet length divided by thesidewall length, is greater than 3.

Another aspect of the present invention relates to a reflectivediffraction grating device on a planar waveguide platform for use inmultiplexing or demultiplexing optical channels defined by an averagewavelength, comprising:

a reflective diffraction grating, which includes a plurality ofreflective walls defined by a facet length; and a plurality ofnon-reflective sidewalls defined by a sidewall length;

an input port for launching a beam of light comprising the opticalchannels at the diffraction grating at an incident angle;

a first output port for outputting one of the optical channels; and

a second output port for outputting another of the optical channels;

wherein the facet length and the incident angle are selected to ensurethat the grating provides diffraction in an order with an absolute valueof 7 or less.

Another feature of the present invention provides a reflectivediffraction grating device on a planar waveguide platform for use indemultiplexing an optical signal, comprising:

an input port for launching a beam of light comprising a plurality ofwavelength channels, defined by an average wavelength, at a diffractiongrating incident angle;

a reflective diffraction grating for dispersing the wavelength channelsat various angles according to wavelength, the reflective diffractiongrating having a plurality of reflective walls defined by a facetlength, and a plurality of sidewalls defined by a sidewall length; and

a plurality of output ports positioned to capture the wavelengthchannels;

wherein the sidewall length is less than or equal to twice the averagewavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 illustrates a conventional reflective diffraction grating;

FIG. 2 illustrates a planar waveguide reflective diffraction gratingaccording to the present invention; and

FIG. 3 illustrates an optical device incorporating the planar waveguidereflective diffraction grating according to the present invention.

DETAILED DESCRIPTION

One of the major concerns in the design of planar waveguide diffractiongratings is the manufacturability of the reflecting and sidewall facetsF and S, respectively. Furthermore, a major limit to themanufacturability of the facets heretofore, has been thephotolithographic resolution limitations. Typical photolithographicprocedures are limited to resolutions in the range of 0.5 to 1.0 μm, sothe minimal requirement to achieve reasonable performance from a gratingis that the reflecting facet size F must be larger than this resolution,say 2.5 to 5 μm or more in size.

In FIG. 1, the light path is simplified by the assumption that the inputand output angles θ_(in) and θ_(Nout), respectively are identical. Thisassumption is only to simplify the mathematical treatment of the facetgeometry. Accordingly:F≈Λ cos θ_(in); and  (6)

Equation (1) simplifies tomλ≈2Λ sin θ_(in)  (7)

Combining equations 6 and 7 yields $\begin{matrix}{F \approx \frac{m\quad\lambda}{2\quad\tan\quad\theta_{in}}} & (8)\end{matrix}$

From FIG. 1: $\begin{matrix}{\frac{S}{F} \approx {\tan\quad\theta_{in}}} & (9)\end{matrix}$

Historically, incidence and output angles of 45° to 65° have been usedinevitably leading to grating facet aspect ratio of F/S to be about 1(see FIG. 1 and Equation 9). At a wavelength of 1550 nm, one finds fromequation (6) that facet sizes, for both reflecting F and non-reflectingsurfaces S, of 10-17 μm are easily achievable in the prior art, for DWDMapplications. This makes grating facets F manufacturable, but at theexpense of large non-reflecting facets (or sidewalls) S contributing tothe polarization dependent loss. In the prior art, facet size variationis also done by varying the diffraction order m, i.e. adjusting thenumerator of equation (8).

Telecommunications networks have evolved from DWDM to CWDM and FTTHnetworks. The latter two network architectures have channels spanninglarge wavelength ranges, from ˜1250 nm to ˜1630 nm. These wide rangescannot be served by a high-diffraction order device, and often requireorders as low as 1. Practitioners of the prior art have not been awareof, or taken advantage of equation (8). At low diffraction orders m andoperating angles θ_(in) and θ_(out) of 45° to 65° the resulting facetsize F for a planar waveguide diffraction grating would be too small tobe practically manufacturable. Existing planar waveguide diffractionbased devices include AWGs and echelle gratings. Both rely on highdiffraction orders; the AWGs need high order operation for guide routingreasons, the echelle technique employs high orders to maintain largefacet sizes that are more easily manufactured. Hence, prior art hasintrinsic limitations in addressing the CWDM or FTTH networkarchitectures in a planar waveguide platform.

The present invention recognizes the importance of equation (8), inparticular the fact that it is possible to increase the grating facetaspect ratio F/S through angular dependence of the denominator. As thediffraction angle is reduced, the facet size increases linearly withtanθ_(in). Additionally, inventors recognize that the increase of thefacet aspect ratio F/S yields devices with improved polarizationdependent loss and larger free spectral range.

For example, in silica-on-silicon, a diffraction order of 5 or less(yielding the smallest practical free spectral range for CWDM or FITHnetworks), at a wavelength of 1550 nm, and size of reflecting facet F toexceed 5.0 μm, would require F/S to be increased to more than 3, whichcan be accomplished by lowering the diffraction angle to about 25°.Thus, the present invention encompasses all planar waveguide diffractiongrating designs with the ratio of reflecting to non-reflecting facets(or sidewalls) of at least 3.

The amount of PDL is strongly dependent on the aspect ratio F/S and thelength of the non-reflecting facet S. Conventional echelle designs havean aspect ratio of ˜1, and are strongly subjected to sidewall dependentPDL; however, for F/S in excess of 3, the non-reflecting facets makesubstantially smaller contribution to the PDL. By further increasingF/S, it is possible to design manufacturable facets with thenon-reflecting grating facet sizes S at or smaller than the wavelengthof the reflected light, e.g. S≦300 nm, preferably ≦2500 nm, even morepreferably ≦2000 nm, and ultimately preferably ≦1550 nm. For suchgratings, the interaction length of light with the metallized sidewallis so small that PDL-free operation of the device becomes possible.

Therefore, when we enter a regime in which tan(θ) is small, i.e. toachieve a ⅓ ratio or θ<25°, we can reduce sidewall dependent PDL.

From a manufacturability standpoint, if reflecting facets F are large,the facets themselves are reproduced faithfully despitephotolithographic resolution limits. Small non-reflecting facets S willlikely not be reproduced faithfully, and will be slightly rounded, butgrating performance is not affected. Practitioners of prior art no doubthave realized that the pitch governs dispersion as per equation (1).However, it is quite common to equate the pitch of a grating to thenormal distance between reflecting facets (the sidewall S in FIG. 1).With that thinking, a distortion to the sidewall S could be equated witha distortion to the pitch. This is a mistaken conception, and in factthe pitch is given by equation (6). Counter-intuitively, the pitchincreases with F, not S. The present inventors recognize this fact andcan increase the aspect ratio, i.e. decrease S/F, shown in equation (9)without risk of affecting the pitch. In fact, the fidelity of thegrating reproduction is limited not by photolithography but by theaccuracy of the features on the mask itself. This limit is severalorders of magnitude (100-fold) smaller than the photolithographicresolution.

Combining equation (8) and (9), we find that: $\begin{matrix}{S \approx \frac{m\quad\lambda}{2}} & (10)\end{matrix}$

Thus, by choosing a small diffraction order (m=3, 2 or 1, if necessary)one can nearly eliminate PDL, because the sidewall size S becomes lessthan the wavelength.

In a preferred embodiment, illustrated in FIG. 3, a concave reflectivediffraction grating 10 is formed at an edge of a slab waveguide 11provided in chip 12. An input port is defined by an end of a waveguide13, which extends from an edge of the chip 12 to the slab waveguide 11for transmitting an input wavelength division multiplexed (WDM) signal,comprising a plurality of wavelength channels (λ₁, λ₂, λ₃ . . . ),thereto. The diffraction grating 10, as defined above with reference toFIG. 2, has an aspect ratio (F/S) greater than 5, and a sidewall lengthS less than or equal to the average wavelength of the wavelengthchannels (λ₁, λ₂, λ₃ . . . ). The input waveguide 13 is positioned toensure that the incident angle θ_(in) is less than 30°, and the gratingpitch Λ is selected to ensure that the grating 10 provides diffractionin an order of 5 or less. The diffraction grating 10 disperses the inputsignal into constituent wavelengths and focuses each wavelength channelon a separate output port in the form of an output waveguide 15, theends of which are disposed along a focal line 16 of the grating 10defined by a Rowland circle, for transmission back to the edge of thechip 12. The illustrated device could also be used to multiplex severalwavelength channels, input the waveguides 15, into a single outputsignal transmitted out to the edge of the chip 12 via the inputwaveguide 13. The input and output ports represent positions on the slabwaveguide 11 at which light can be launched or captured; however, theports can be optically coupled with other transmitting devices or simplyblocked off.

Specific examples for operating the aforementioned optical device are:θ_(in =) 5° 5° 5° 6° m = 1 2 3 2 λ_(avg) = 1550 nm 1550 nm 1550 nm 1550nm Λ = 8892 nm 17784 nm 26676 nm 14828 nm F = 8858 nm 17716 nm 26574 nm14747 nm S = 775 nm 1550 nm 2325 nm 1550 nm F/S = 11.4 11.4 11.4 9.5

1. A reflective diffraction grating device on a planar waveguideplatform for use in demultiplexing an optical signal, comprising: aninput port for launching a beam of light comprising a plurality ofwavelength channels, defined by an average wavelength, at a diffractiongrating incident angle; a reflective diffraction grating for dispersingthe wavelength channels at various angles according to wavelength, thereflective diffraction grating having a plurality of reflective wallsdefined by a facet length, and a plurality of sidewalls defined by asidewall length; and a plurality of output ports positioned to capturethe wavelength channels; wherein an aspect ratio, defined by the facetlength divided by the sidewall length, is greater than
 3. 2. The deviceaccording to claim 1, wherein the aspect ratio is greater than
 5. 3. Thedevice according to claim 1, wherein the aspect ratio is greater than10.
 4. The device according to claim 3, wherein the diffraction gratingincident angle is less than 6°.
 5. The device according to claims 3,wherein the sidewall length is less than or equal to two times theaverage wavelength.
 6. The device according to claim 1, wherein thesidewall length is less than or equal to the average wavelength.
 7. Thedevice according to claim 1, wherein the incidence angle less than 30°.8. The device according to claim 1, wherein the incidence angle is lessthan 15°.
 9. A reflective diffraction grating device on a planarwaveguide platform for use in multiplexing or demultiplexing opticalchannels defined by an average wavelength, comprising: a reflectivediffraction grating including: a plurality of reflective walls definedby a facet length; and a plurality of non-reflective sidewalls definedby a sidewall length; an input port for launching a beam of lightcomprising the optical channels at the diffraction grating at anincident angle; a first output port for outputting one of the opticalchannels; and a second output port for outputting another of the opticalchannels; wherein the facet length and the incident angle are selectedto ensure that the grating provides diffraction in an order with anabsolute value of 7 or less.
 10. The device according to claim 9,wherein the order is 5 or less.
 11. The device according to claim 9,wherein the order is 3 or less.
 12. The device according to claim 11,wherein the diffraction grating incident angle is less than 6°.
 13. Thedevice according to claim 12, wherein the sidewall length is less thanor equal to the average wavelength.
 14. The device according to claim 9,wherein the incidence angle is less than 45°.
 15. The device accordingto claim 9, wherein the incidence angle less than 30°.
 16. The deviceaccording to claim 9, wherein the incidence angle is less than 15°. 17.The device according to claims 9, wherein the sidewall length is lessthan or equal to two times the average wavelength.
 18. The deviceaccording to claims 9, wherein the sidewall length is less than or equalto the average wavelength.
 19. A reflective diffraction grating deviceon a planar waveguide platform for use in demultiplexing an opticalsignal, comprising: an input port for launching a beam of lightcomprising a plurality of wavelength channels, defined by an averagewavelength, at a diffraction grating incident angle; a reflectivediffraction grating for dispersing the wavelength channels at variousangles according to wavelength, the reflective diffraction gratinghaving a plurality of reflective walls defined by a facet length, and aplurality of sidewalls defined by a sidewall length; and a plurality ofoutput ports positioned to capture the wavelength channels; wherein thesidewall length is less than or equal to twice the average wavelength.20. The device according to claim 19, wherein the sidewall length isless than or equal two the average wavelength.